The regulation of cyclic nucleotide phosphodiesterase activity constitutes a potential mechanism for modulating intracellular concentrations of cyclic AMP and cyclic GMP. Multiple forms of cyclic nucleotide phosphodiesterase have been distinguished in liver, brain and other tissues. The activity of the enzymes has been shown to be altered in vitro by metals, cyclic nucleotides, a heat-stable protein activator molecule, protein kinase activity and various pharmacologically active compounds. The objective of this study is to better delineate the inter-relationships of these and other regulatory factors by purifying the low and high Km cyclic AMP phosphodiesterases and cyclic GMP-phosphodiesterases of rat liver free of contaminating protein kinase activity, protein kinase modulator and phosphodiesterase activator. The enzymes will be characterized during the following studies which involve: 1) purifying and studying the properties of a heat-stable factor (cyclic GMP regulatory factor) which may control the activity of cyclic GMP to either stimulate or inhibit the hydrolysis of cyclic AMP; 2) determining the mechanism by which imidazole differentially inhibits or stimulates the hydrolysis of both cyclic AMP and cyclic GMP by purified phosphodiesterases; 3) determining the mechanism by which various pharmacological agents differentially inhibit the hydrolysis of cyclic AMP and cyclic GMP by the individual enzymes.